The present invention relates to an integrated circuit for image signal processing, and more particularly, to a synchronizing signal clamping circuit which enables the integration of a normally external capacitor into the internal chip circuitry of the integral image signal processing circuit.
Signal processing circuit integration has progressed rapidly as video equipment such as VCRs and camcorders have become smaller and lighter in weight. In this field, integrated circuits are generally divided into those for luminance signal processing and those for chrominance signal processing. However, recently, the circuitry for both luminance and chrominance signal processing has been integrated into a single chip. Since an image signal is an analog signal, passive elements, e.g., capacitors and resistors, are required for the image signal processing integrated circuit. Capacitors and resistors having relatively large values occupy excessive chip area. Therefore, such elements cannot be integrated into the chip internally but can be used if connected as an external passive component.
As will be appreciated by one of ordinary skill in the art, whenever external elements are utilized in conjunction with integrated circuitry, the number of pins of the integrated circuit must be increased. Thus, a larger circuit board layout area is required and product assembly time increases, which ultimately increases cost. Moreover, it is desirable to integrate external elements onto the chip in order to increase the number of pins available for performing additional functions.
In a conventional synchronizing signal keyed clamping circuit of an integrated image signal processing circuit, a capacitor is used to detect an error voltage during a "high" interval of the synchronizing signal. This function requires a large capacitance, such that the capacitor cannot be integrated into the chip. Therefore, a large external capacitor, e.g., 4.7 .mu.F, is connected to the synchronizing signal keyed clamping circuit via pins of the integrated image signal process circuit. This increases the required number of pins and increases the cost of chips including the conventional synchronizing signal keyed clamping circuit. Further, the external placement of the passive component unfavorably expands the circuit board layout.